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Abstract

We demonstrate the use of a double-beam optical tweezers system to stabilize red blood cell (RBC) orientation in the optical tweezers during measurements of elastic light scattering from the trapped cells in an angle range of 5-30 degrees. Another laser (He-Ne) was used to illuminate the cell and elastic light scattering distribution from the single cell was measured with a goniometer and a photomultiplier tube. Moreover, CCD camera images of RBCs with and without laser illumination are presented as complementary information. Light scattering from a RBC was measured in different fixed orientations. Light scattering from cells was also measured when the length of the cell was changed in two different orientations. Light scattering measurements from spherical and crenate RBCs are described and the results are compared with other cell orientations. Analysis shows that the measured elastic light scattering distributions reveal changes in the RBC’s orientation and shape. The effect of stretching on the changes in scattering is larger in the case of face-on incidence of He-Ne laser light than in rim-on incidence. The scattering patterns from RBCs in different orientations as well as from a spherical RBC were compared with numerical results found in literature. Good correlation was found.

CCD camera images from different trapped objects: (a) light scattering from a 6.0 µm sphere, (b) image of a swollen RBC, (c) light scattering distribution from a spheroid cell (in figure (b)), (d) image of an RBC in a hypertonic environment, and light scattering from the same cell (e). The arrow shows the direction of the incident laser light. Scale bar is 10 µm.

Light scattering signal from a polystyrene sphere and a spherical red blood cell. The polarization directions of the incident He-Ne laser beam are shown in brackets. The thick black line depicts scattering from the RBC in a hypertonic suspension. Mie theory modelings of the polystyrene sphere and spherical RBC are included in the figure. The small inset shows the repeatability of the measurements during three different measurements.

Light scattering distributions from red blood cells in different orientations. Differential image of a cell in a rim-on case (a), and differential image of the cell in a face-on case (b). Arrows show the direction of the incident laser beam. Scale bar is 10 µm.